It is quite clearly nute burn. Flush straight away.Then feed only water for a few days and 400ppm thereafter with an increase up to 500ppm if they are still happy, so on and so on.
N Def affects the whole leaf gradually fading in colour until totally see through yellow. Nute probs cause yellowing/browning of perimeters of leaves and burnt looking, little patches missing almost like its been munched.
Lots of things turn leaves yellow but that does look like nute burn. Folow woomeiters instructions and dont overwater that could be a problem too. Yelow/brown leaves are closer to death and can be caused by nute burn.